If a vehicle is rear-ended or the vehicle collides at the rear end while reversing (i.e., what is called a rear-end collision), the head of an occupant seated on a vehicle seat tilts backward due to inertia, with the result that the neck of the occupant may receive an impact.
In order to protect the head and the neck of the occupant from an impact caused by a rear-end collision and thus reduce the impact on the neck, the vehicle seat has a headrest which is provided at an upper portion of the seat back and designed to receive the head of the occupant at the rear side. To efficiently reduce the impact in the rear-end collision, it is desirable to quickly reduce a gap between the head of the occupant and the headrest in the rear-end collision.
For this purpose, there has been suggested a seat back configured to allow the upper body of the occupant to be moved backward in a rear-end collision, to receive the load generated by this movement by a pressure-receiving member and to cause the headrest to move frontward, and to support the head of the occupant to thereby reduce the impact on the neck of the occupant (see Patent Literature 1).
The seat back frame of the vehicle seat disclosed in Patent Literature 1 has right and left side frames which are connected with each other at their lower portions by a plate-like lower frame (lower connecting member) and a rotary supporting shaft for seat reclining which is disposed frontward of the lower frame. The lower frame is not limited to one having a plate-like shape; as disclosed in Patent Literature 2, the lower frame may have frontwardly extending flanges at upper and lower ends thereof. These lower frames are oriented to a substantially vertical attitude when the occupant sits in a normal seating posture, such that it is slightly faced down with respect to the upper portion of the seat back (where an upper region of the back of the occupant contacts).